The field of the invention is nuclear magnetic resonance imaging methods and systems. More particularly, the invention relates to phase contrast angiography.
When a substance such as human tissue is subjected to a uniform magnetic field (polarizing field B.sub.0), the individual magnetic moments of the spins in the tissue attempt to align with this polarizing field, but precess about it in random order at their characteristic Larmor frequency. If the substance, or tissue, is subjected to a magnetic field (excitation field B.sub.1) which is in the x-y plane and which is near the Larmor frequency, the net aligned moment, M.sub.z, may be rotated, or "tipped", into the x-y plane to produce a net transverse magnetic moment M.sub.t. A signal emitted by the excited spins may be received after the excitation signal B.sub.1 is terminated and processed to form an image.
When utilizing these signals to produce images, magnetic field gradients (G.sub.x G.sub.y and G.sub.z) are employed. Typically, the region to be imaged is scanned by a sequence of measurement cycles in which these gradients vary according to the particular localization method being used. The resulting set of received NMR signals are digitized and processed to reconstruct the image using one of many well known reconstruction techniques.
Most NMR scans currently used to produce medical images require many minutes to acquire the necessary data. The reduction of this scan time is an important consideration, since reduced scan time increases patient throughput, improves patient comfort, and improves image quality by reducing motion artifacts. There is a class of pulse sequences which have a very short repetition time (TR) and result in complete scans which can be conducted in seconds rather than minutes. When applied to cardiac imaging, for example, a complete scan from which a series of images showing the heart at different phases of its cycle can be acquired in a single breath-hold.
The number of cardiac phase images that can be acquired during a scan is determined by a number of factors such as pulse sequence repetition time, the number of views acquired at each cardiac trigger and the patient's heart rate. By decreasing the number of views acquired at each cardiac trigger, more phase images can be acquired and the "temporal resolution" of the series of cardiac images is increased. However, total scan time is increased as the number of views acquired per cardiac trigger is reduced and the number of cardiac triggers required to complete acquisition of an image with the same spatial resolution increases.
The same temporal resolution problem exists when flow encoded acquisitions are made to produce a magnetic resonance angiogram. In addition to the different phase encoded views required to reconstruct each image, at least two flow encoded views are required at each phase encoding in order to reconstruct an angiographic image. As used in this patent, the term "view" encompasses both phase encoding view and flow encoding view. If flow is to be measured along all three axes, a minimum of four flow encoding views are required at each phase encoding, and the task of providing high temporal resolution within a short scan time is even more difficult.